Seems that opening for example http://safebooru.donmai.us/posts.json?limit=15&page=1 as anonymous user gives an error, `wrong number of arguments (given 1, expected 0)`. There's no stack trace, just this message and this doesn't happen for logged in user.
Haven't tested any other API functionality, since that's pretty much the only part that I use...
Edit:
Nvm, didn't notice it was already reported in the "issues" thread.
Updated